1. Field of the Invention
The present invention relates to a start-up circuit for controlling the sequence on start-up of electronic equipments and the like.
2. Description of the Related Art
A great number of electronic circuits such as CPU (Central Processing Unit) and other DSP (Digital Signal Processor) which perform digital signal processing, liquid crystal panel and other analog circuits are mounted on recent various electronic equipments such as a portable telephones, a PDA (Personal Digital Assistant), a laptop computer. The electronic circuits operate with power supplied from a battery or a power supply circuit for stabilizing the battery voltage.
A stable operation guaranteed voltage is defined for each electronic circuit, and the relevant electronic circuit does not operate properly if the supplied voltage is equal to or lower than the stable operation guaranteed voltage. Therefore, an under voltage lock out (hereinafter referred to as UVLO) function of monitoring the battery voltage etc. and controlling the start-up and terminating sequences of each electronic circuit is installed in such electronic equipment. Related arts are disclosed in Japanese Patent Application Laid-Open Nos. 2004-22947 and 2004-126922 and the like.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-22947    [Patent Document 2] Japanese Patent Application Laid-Open No. 2004-126922    [Patent Document 3] Japanese Patent Application Laid-Open No. 2003-209461
The UVLO circuit compares the voltage to be monitored (hereinafter also referred to as monitored voltage) such as the battery voltage with a predetermined threshold voltage, executes a predetermined start-up sequence when the battery voltage becomes higher than a threshold voltage, and executes a predetermined terminating sequence when the battery voltage becomes lower than the threshold voltage. A circuit, which executes the predetermined sequences (hereinafter referred to as start-up circuit in the present specification), is built in the UVLO circuit. The start-up circuit executes a predetermined event at a predetermined order and timing when the UVLO circuit permits starting of start-up.
The start-up circuit needs to measure the time in order to determine the timing of executing the predetermined event. A method for generating a clock signal using an oscillator and counting the clock signals up to a predetermined number is generally used as a method for measuring the time. However, the oscillator uses a lot of electricity so that it may have hindered reducing the power consumption.